Adhesives

Adhesives

An adhesive is a substance that sticks to the surface of an object such that two surfaces become bonded. A typical home improvement store carries many different adhesives for many different applications. Why are there so many adhesives? The answer is found in examining how an adhesive works and, in particular, what happens at the molecular level. The interaction of molecules is known as intermolecular bonding, or secondary bonding. Primary bonding, also known as intramolecular bonding, is the interaction of atoms within a molecule and includes covalent and polar covalent bonding. Secondary bonding includes dipole–dipole bonding (the interaction of molecules that have a permanent net dipole moment) and hydrogen bonding (an interaction that occurs when a hydrogen atom is bonded to an N, O, or F atom in a molecule).

Adhesives cure when the small resin molecules (mers) join together to form extremely large molecules known as polymers. For example, one of

the simplest polymers is polyethylene. The mer (basic building block of the polymer) is ethylene, H2C=CH2. The addition of an initiator (R·) causes the formation of the radical RCH2CH2·. A radical is a species that has an unpaired electron and is very reactive because it seeks the source of electrons. This radical will attach the ethylene mer (the double bond in ethylene is rich in electrons) to start a chain reaction that continues until very large polymer molecules form. This and other forms of polymerization processes are the basis for the formulation of polymers. This process is known as curing when dealing with adhesives.

Two criteria must be met in order for a molecule to possess a permanent net dipole moment: (1) an unequal sharing of electrons within the molecule such that one or more intramolecular bonds has a partial positive end and a partial negative end, and (2) a geometry such that the vector sum of the individual dipole moments does not equal zero. The ability of an atom within a molecule to attract electrons is known as electronegativity, a concept proposed by Linus Pauling who established a table of relative electronegativities. In Pauling's table, fluorine is the most electronegative element and is given the value of 4.0. The greater the difference in electronegativity between two atoms within a molecule, the larger is the dipole moment in that bond. Because the bond between two atoms having unequal electronegativities has a partial positive end and a partial negative end, it is said to be a polar bond. If the geometry of the molecule is such that the vector sum of all of the dipole moments does not equal zero, then the molecule is polar. The electronegativities for carbon and oxygen are 2.5 and 3.5, respectively; therefore, the carbon–oxygen bond is a polar bond. A carbon dioxide molecule has two carbon–oxygen bonds; however, its geometry is such that the vector sum of the two dipole moments equals zero, and thus carbon dioxide is a nonpolar molecule. The electronegativity of hydrogen is 2.1, thus a hydrogen–oxygen bond would be polar. A water molecule has two hydrogen–oxygen bonds. The geometry of a water molecule (the H–O–H bond angle is 104.5°) is nonsymmetrical, hence the vector sum of the dipole moments is not equal to zero and water is a polar molecule.

Polar molecules will attract other polar molecules because of their net dipole moments. Water molecules, however, have an additional attraction for one another, based on hydrogen bonding. This attraction is so strong that, although water is a small molecule and small molecules tend to be gases, water is a liquid at room temperature. This aspect of the chemistry of water demonstrates that hydrogen bonding is a relatively strong force that can hold molecules together.

So how does this relate to the many types of adhesive products that exist? In order for an adhesive to bond (hold together) two surfaces (substrates), there must be several types of interaction between the adhesive and both substrates. The first type of interaction is that the adhesive must wet the substrate, meaning that the adhesive must spread itself out into a film that covers the substrate surface. In order for this to happen, the adhesive must have a low enough viscosity so that it will flow. Viscosity is the resistance of a liquid to flow. Water has a low viscosity whereas honey has a high viscosity. Because viscosity is temperature dependent, the application of a cold adhesive to a substrate, or the application of an adhesive to a cold substrate, may result in poor wetting. Another factor that affects wetting is the relative strengths of cohesive forces (between like molecules, such as two adhesive molecules) and those of adhesive forces (between unlike molecules, such as an adhesive molecule and a substrate molecule). If the cohesive forces among adhesive molecules are weaker than the adhesive forces between the adhesive molecules and the substrate surface, then the adhesive molecules will spread out over the substrate and wet its surface. An adhesive that has a relatively low viscosity and is able to wet the substrate surface will flow into any tiny cracks or pores on the substrate surface, thus promoting what is known as mechanical bonding. Mechanical bonding increases the strength of an adhesive bond and, as a result, a forced separation of the two substrate surfaces is more apt to tear the substrate surfaces.

Mechanical bonding is one of several ways that an adhesive bonds substrates. All surfaces, except those that are highly polished, have pores. If the adhesive flows into these pores and then polymerizes, a mechanical bond is formed. It is similar to placing a wick into liquid candle wax. Once the wax solidifies the wick can not be easily removed. A mechanical bond has formed.

Because the interactions of adhesive molecules with substrates are so critical, it makes sense that some adhesives would be more appropriate for a specific substrate than others. Adhesives are designed for specific applications. For example, adhesives known as "super glues" (cyanoacrylates) are useful around the home in the bonding of common substrates (e.g., dishes, toys, etc.), which can take place in a matter of seconds. Yet their usefulness is limited when bonding wood because the cure time (the time it takes for an adhesive to undergo polymerization and become capable of holding the two substrates together) in this instance is much longer. Cyanocrylates tend to be brittle thus they are vulnerable to impact and dramatic changes in temperature. To reduce these shortcomings, small amounts of finely ground rubber has been used as filler. The rubber introduces flexibility thus reducing brittleness. In addition, cyanocrylates are attacked by polar solvents. Polar solvents will weaken cured cyanocrylate bonds over time. Therefore, applications involving water, alcohols, or other polar solvents should be avoided. Polymerization is the reaction of small molecules combining to form very

TYPES OF ADHESIVES

Adhesive Type

Sources/Properties

Common Uses

Animal glue

Obtained from animal byproducts such as bones, blood, and hooves

Binding of abrasives in sandpaper and other grinding materials

Casein

Main protein in milk

Labels on beer bottles that do not come off in ice water, yet are recyclable

Starch

From corn and maize

Corrugated cardboard bonding

Natural rubber

Not "sticky enough" by itself but is used as an additive in other adhesives

Self-adhesive envelopes and other pressure-sensitive adhesives; adhesives that bond to substrates on contact (like tapes)

Bonding of layers in plywood and the bonding of particles in particle board

Polyurethane

A flexible adhesive

Bonding soles to the bodies of shoes; also used in food packaging

Polyvinyl acetate

Common "white" glue

Book bindings and labels

Polyolefin/ethylene copolymer

No solvents involved

Hot melts

Acrylates or anaerobic adhesives

Cure when air is removed

Adhesive used to keep nuts tight on bolts, such as those within ATMs and heavy machinery

Silicone

Both an adhesive and a sealant and only common adhesive that is based on silicon rather than carbon

Bathtub and shower sealants; also many car applications, such as oil pans and head gaskets

large molecules. The reason for the longer cure time in the bonding of wood has to do with chemical reactions (between adhesive and substrate) that affect the rate at which the cyanoacrylate molecules polymerize. A typical cyanoacrylate product is a low viscosity liquid that readily flows over a substrate's surface. When the cyanoacrylate molecules encounter a basic environment, the small molecules within the adhesive polymerize. Many common surfaces tend to be basic, so a cyanoacrylate adhesive works fine in these instances; wood, however, tends to have an acidic surface, and because an acidic environment will inhibit polymerization, cure time is increased.

Cyanoacrylates are not appropriate for the bonding of the steel parts of an automobile, because of the environments that the car will be exposed to. Those environments include such things as rain, variations in temperature, exposure to solvents (such as gasoline, oil, and windshield washer solution), ozone, acid rain , salt spray, and ultraviolet light from the Sun. (A more appropriate adhesive for car parts would be an epoxy-based adhesive.) Another example of a "special" adhesive would be the one used to attach a new rearview mirror in an automobile. Because the cured adhesive in this case will be exposed to wide variations in temperature and to an extremely large amount of ultraviolet light from the Sun for prolonged periods of time, an adhesive formulated specifically for these conditions should be used.

Finally, the strength and permanence of the bond formed between adhesive and substrate must be considered when one is selecting an adhesive. Most of the time it is desirable to have maximum strength and permanence; the very common Post-it note, however, is a counterexample. Its adhesive is neither strong nor permanent. The adhesive formulation that eventually led to the Post-it was initially considered a failure because of the adhesive's weakness. It was while singing in his church choir that Arthur Fry of the Minnesota Mining & Manufacturing Company (3M) envisioned a potential use for what seemed to be a useless material.

Table 1 lists some common types of adhesives and their uses. Because of the different possible substrates and combinations of substrates, and because adhesives are subject to such a range of environmental conditions, it is no wonder that there are so many types of adhesives on the market. However, if one has some knowledge of how adhesives bond to substrates and the types of substrates being bonded, the task of selecting adhesives will not be overwhelming.

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Adhesives

UXL Encyclopedia of Science
COPYRIGHT 2002 The Gale Group, Inc.

Adhesives

Adhesives bond two or more materials at their surfaces. Any time you reattach a handle that has broken off a cup, paste a photograph into an album, or tape a message to a friend's door, you use an adhesive.

Natural adhesives such as beeswax, resin (tree sap), and bitumen (asphalt) have been used since earliest times. Ancient Egyptians used flour paste in the making of papyrus (reed paper) and glue made from animal skin and bones for woodworking. Monks of the Middle Ages (400–1450) used egg white to glue gold leaf to their illuminated (decorated) manuscripts.

Flour paste, animal glue, and egg white are examples of natural adhesives. Natural adhesives are still widely used but have been replaced for most applications by synthetic adhesives. Synthetic adhesives are compounds invented by chemists with special properties that make them useful for various kinds of bonding. For example, two metals can be bonded to each other using an epoxy resin, a type of plastic. Or metal can be bonded to wood using a rubber like material known as neoprene.

New adhesives and new applications

Researchers are constantly looking for new kinds of adhesives and new ways to use them. For example, engineers at European Airbus, one of the world's largest manufacturers of airplanes, has found that plasticlike adhesives can be used to hold airplane sections together. The adhesives are cheaper and easier to use than traditional welds and rivets.

A better adhesive is not always a stronger adhesive. The material used on Post-it™ notes is an example. This adhesive is just strong enough

to hold a Post-it™ note to another piece of paper, wood, glass, or plastic. But it comes loose easily and can be reattached many times. Post-it™ notes were created in 1974 by Arthur Fry, a chemist at the Minnesota Mining and Manufacturing Company (3M). Fry made use of a "failed" discovery by a colleague who was trying to make a very strong adhesive, instead producing a very weak one.

One of the most widely used forms of adhesives is adhesive tape. Adhesive tape consists of an adhesive that has been attached to paper, cloth, rubber, plastic, or some other material. The familiar Scotch tape™ is perhaps the best known of all adhesive tapes. Scotch tape™ was invented in the 1920s by Richard Drew, another chemist at the 3M company. Drew found a way to coat Du Pont's newly invented cellophane tape with a thin layer of adhesive to make it the all-purpose aid present in most households today. Variations of the original Scotch tape™ in use today are carpet tape, masking tape, duct tape, and sealing tape.

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adhesive

The Columbia Encyclopedia, 6th ed.

Copyright The Columbia University Press

adhesive, substance capable of sticking to surfaces of other substances and bonding them to one another. The term adhesive cement is sometimes used in place of adhesive, especially when referring to a synthetic adhesive. Animal glue, a gelatin made from hides, hooves, or bones, was probably known in prehistoric times; it remained the leading adhesive until the 20th cent. It is now used especially in cabinetmaking. Animal glue is sold both as a solid (either ground or in sheets, to be melted in a water-jacketed glue pot and applied while hot) and as liquid glue (an acidic solution). Adhesives from vegetable sources are also important; they include natural gums and resins, mucilage, and starch and starch derivatives. They are commonly used for sizing paper and textiles and for labeling, sealing, and manufacturing paper goods. Other adhesives derived from animal and vegetable sources include blood glue, casein glue, fish glue, rubber adhesives, and cellulose derivatives. Adhesives having special properties are prepared from synthetic resins. Some synthetic adhesives, such as the epoxy resins, are strong enough to be used in construction in place of welding or riveting. Adhesive tapes have a coating of pressure-sensitive adhesive.

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adhesion

ad·he·sion
/ adˈhēzhən/
•
n.
1.
the action or process of adhering to a surface or object:
the adhesion of the Scotch tape to the paper. ∎
the frictional grip of wheels, shoes, etc., on a road or other surface:
the front tires were struggling for adhesion. ∎ Physics
the sticking together of particles of different substances.
∎
allegiance or faithfulness to a particular person, party, or set of ideas:
his adhesion to the old bureaucracy.2. Med.
an abnormal union of membranous surfaces due to inflammation or injury.

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adhesion

adhesion (ăd-hee-zhŏn) n.1. the union of two normally separate surfaces by fibrous connective tissue developing in an inflamed or damaged region. (The fibrous tissue itself is also called an adhesion.) Adhesions between loops of intestine often occur following abdominal surgery but only rarely cause symptoms, such as intestinal obstruction.

2. a healing process in which the edges of a wound fit together.

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adhesion

adhesion In medicine, fibrous band of connective tissue developing at a site of inflammation or damage; it may bind together adjacent tissues, such as loops of intestine, occasionally causing obstruction. Most adhesions result from inflammation or surgery.

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